my horse speed up after jumps too and what you could do is half halt, when he slows his pace down you release a little. also working on downward transitions such as asking to walk or halt after a jump helps, don't forget to use your voice too!
Halts after every jump , this way the horse will learn not to run in front and after an obstacle , being more calm and confident and the most important i think , to take big turns , never rush with them
I do recommend much of gymnastics from trot with easy distances for the horse varying the type of jumps and starting low ! This will make him less anxious and will teach him to slow down and trust in himself more in fact it helps you too as a rider
Work on courses at home, doing halts or downward transitions after a jump, so he learns that there's no need to speed up cause he's going to halt anyway. When you get that practice on tight turns, so he needs to slow down to stay in balance.